1. Field of the Invention
The present invention relates to an improvement for reinforcing bead sections of radial tires provided with carcass ply composed of steel cords, radial tires for heavy-duty vehicles, such as trucks and buses, etc.
2. Description of the Prior Art
In heavy-duty vehicles there has recently been used radial tires, each having a rigid reinforcement in its tread portion, particularly a belt reinforcement composed of metal cords, such as steel cords or the like.
In general, radial ply tires using metal cords as a belt reinforcement have several advantages, such as wear resistance, puncture resistance and the like, as compared with the usual bias ply tires. This is because a stiff belt is disposed between the tread rubber and the carcass ply, but there is a disadvantage due to the rigid reinforcing effect of the belt.
The development of such radial tires are particularly advanced for use on good, smooth roads, as distinguished from rough or bad roads. Lately, the demand for these tires has increased because of the remarkable improvement of roads, even in more remote locations.
In general, a tire of this type comprises, as shown in FIG. 1, a carcass 2 of a single ply arranged in a substantially radial direction of the tire, or semi-radially of the tire, wherein the carcass ply forms small angle of about 10.degree.-20.degree. and at the largest an angle of less than 30.degree., with respect to the mid-circumferential plane of the tire, and is provided with a hard rubber apex strip with JIS hardness greater than 80.degree., and a relatively soft buffer strip 12 with JIS hardness of about 50.degree.-60.degree. disposed radially and axially outwardly from bead core 3 between the carcass main portion 2 and the main turnup portion 2' of the carcass ply 2.
The rubber strips 11 and 12 form a substantially triangular section, with a chafer strip 4 positioned outside the turnup portion 2' of the carcass ply 2. The chafer strip is composed of the rubberized metallic cords, each being crossed at an angle of 30.degree.-60.degree. range with respect to the radial direction of the tire. The aforementioned metallic cord chafer strip is provided for the exclusive purpose of alleviating stress concentration on the cord ends 2a of the turnup portion 2' of the carcass ply 2, and accordingly the radial outer end 4a of chafer strip 4 extends radially outwardly farther than the upper end 2a of the turnup portion 2'.
The axial and radial inward end of the chafer 4 is locked and turned substantially around the bead core 13, extending between the bead core and the bead seating portion.
In the aforementioned bead structure, however, the end 4a of the chafer 4, which is radially outwardly from the turnup portion 2', results in a new concentration of stress when under load, and the consequent repeated flexing of the tire sidewalls combined with the stresses imposed thereon results in early deterioration of the lower sidewalls, such as separation and rupture of the carcass ply and the chafer strip in the lower sidewalls and the bead portion.
To eliminate the above disadvantages, it has been proposed to stiffen the bead sections, and to this end the beads are built up in such a way as to include, in addition to the metallic bead core and the carcass ply wound around it, reinforcements of various kinds, for example, as disclosed in Japanese Pat. No. 976452.
A typical construction of this patent is shown in FIG. 2 hereof, having a chafer 4 composed of metallic cords disposed radially outside the turnup portion 2' of the carcass ply 2 and reinforcing layers 6 composed of rubberized organic fiber cords, wherein two or more layers are crossed with each other, and arranged outside of chafer strip 4 and extend radially outwardly from the bead base to the region at which the width of the tire cross-section is at a maximum when the tire is inflated. The upward cut ends of the turnup portion of the carcass ply and of the metallic cord chafer are completely covered with organic fiber cord reinforcing layers 6, and strips 11, 12 are disposed between the carcass ply main portion 2. The turnup portion 2' with the chafer 4 and the rubberized organic fiber cord reinforcing layers 6 extend radially and axially outwardly from the bead core 3 beyond outermost cord ends of the turnup portion and of the metallic cord chafer in radial direction, and gradually decrease in thickness towards the sidewall of the tire to form a substantially triangular section, which consists of an apex strip 11 of a hard rubber stock with JIS hardness more than 80.degree. and a buffer strip 12 of a relatively soft rubber stock with JIS hardness of 50.degree.-60.degree.. This assembly is along the side of the turnup portion 2' of carcass ply 2 and extends along the organic fiber cord reinforcing layers 6 outwardly in the radial direction of the tire to protect the outermost cords ends 2a, 4a of the turnup portion and the metallic cord chafer, respectively, from stress concentration of them by the consequent repeated flexing of the tire sidewalls.
Another prior art attempt is in U.S. Pat. No. 3,557,860, which discloses a pneumatic radial tire in which the beads comprise at least a pair of bead core of steel wires about which the carcass plies are turned up, said carcass plies being all turned up axially outwardly towards the rim flange, and including a triangular filler made of a high rigidity rubber layer or the like above the bead core, and reinforcing elements constituted by two groups of strips of cord fabric, each group having one or more strips, the first group being placed inwardly of the bead between the carcass plies and the triangular filler, and the second group being placed outwardly of the bead and of the carcass plies and alongside the turned-up portion of said plies, said groups of strips extending radially from a point approximately at the height of the inner periphery of the bead ring to at least one-sixth of the height of the tire section. Tires constructed as above-described have been favorably accepted by drivers as a tire which may be used under severe and heavy load conditions.
Recently, some drivers have begin to use such tires under even more severe service conditions, such as running at high speed for a long period of time under heavy loads. In addition, in order to substantially reduce the cost of tires, the frequency of using a steel core radial tire for recapping has increased. It has become common to desire to recap a worn tire two or three times. Under such circumstances, the above-described structures are usually insufficient as to durability of the bead portion, which is vital to preserve a worn tire for recapping.
Radial and semi-radial tires usually have a carcass of a single ply composed of radially directed cords. As a result, the sidewall of a radial tire is extremely soft, and hence its flexure in the direction of the axis of rotation of the tire is very large. Thus, the amount of deformation of the tire during one rotation is larger than that of the bias tires. That is, the bead section of the said tire is strongly forced to deform over the rim flange in a convex configuration under the influence of such soft sidewalls, and it was found by the inventor that the internal temperature of the bead section increases up to over 120.degree. C. by the transmission of heat from the brake drum of vehicles.
Thus, the bead sections suffer from dynamic fatigue and thermal fatigue. Unfortunately, the adhesion of the rubber with the metallic cords or fabric cords, which is vital to the maintenance of tire performance, is closely related to this dynamic and thermal fatigue.
When the internal temperature of the tire rises above a certain level, the tire will be suddenly broken down by the lowering of the adhesion among these components.
Accordingly, it is necessary and essential for advancement of tire durability to establish techniques which can control the internal termperature of a tire at a low level and which can maintain the adhesion at a high level, which do not so depend upon the internal temperature thereof.
As part of the effort to overcome these prior art problems, the present application has proposed, for example, an excellent reinforced construction of a bead section in Japanese Pat. No. 967452 (Patent Publication No. 11481 of 1977), which is disclosed in FIG. 2 hereof.
As above-mentioned, a tire construction as above-described has been favorably accepted by drivers as a tire which can be used under severe and heavy load conditions.
As above-mentioned, a principal object of the present invention is to control the internal temperature increase and the partial movement of the bead section by way of removing the gap of elasticity in the radial direction, the circumferential direction and the lateral direction from the upper sidewall to the bead portion by arranging on the outer side of the tire, in axial direction thereof, at least one fiber cord reinforcing layer composed of the rubber coated organic fiber cords, like nylon and polyester, having medium elasticity between rubber compounds and steel cords, in order to mitigate stress concentrations and deflection in the cords ends 4a of the steel cord reinforcing layer and the cords ends 2a of the turnup portion 2' of the carcass ply 2.
When one reinforces the bead portion by fiber cords, as above-mentioned, the durability of tires is certainly increased, but the service conditions of tires have become increasingly severe, and since there are types of radial tires which are repeatedly used for recapping the tire treads several times, this recapability is regarded as a sales point for steel radial tires. This recapability depends on the durability of the bead section, and the above-described known tires do not always satisfy this requirement.
On the other hand, these tires have been used under heavy loads and severe conditions and frequently used under the more severe service conditions to save fleet expenses. Therefore, more improvement of the durability of the bead sections is required.
We turn now to a consideration of strain behavior in the bead section when the above-discussed tire is inflated. As shown in FIG. 3, a carcass ply main portion 2 turned up around a bead core 3 moves upwardly, as shown by an arrow 21, and the end 2a of the turnup portion moves downwardly in the radial direction, as shown by arrow 22, thus tending to disengage the carcass ply from the bead core 3, the turnup portion 2' of the carcass ply 2 causes steel cord chafer strip 4 and the organic fiber cord reinforcing layers 6 to move downwardly, as shown by an arrow 23.
The bead core 13 likewise tends to turn, as shown by arrow 24, in addition, the deformation of the tire when it is inflated causes a shearing strain to be produced among those components.
When the adhesion among these layers is unable to endure this strain, the carcass ply 2 is pulled out of the bead core 3, this phenomenon will hereinafter be called "blow out" for the sake of convenience.
This "blow out" phenomenon is apt to occur owing to the said internal temperature, and the higher the temperature, the earlier it occurs.
The inventors have made various investigations with respect to the aforementioned internal temperature and strain in the bead section in order to provide a tire having an improved durability in its bead reinforcing structure and found the following facts in relation to such prior art, that is, having at least two reinforcing organic fiber cord layers and a steel cord reinforcing layer arranged axially outside of the turnup portion of the carcass ply, as proposed in Japanese Pat. No. 967452.
To begin with, the high inner pressure and the high load on the tire are loaded by the carcass ply main portion and the cord paths (the space between the adjacent steel cords) are expanded by the said load at the upper side of the bead portion, then the sidewall portions over the rim flange are forced to deflect largely towards the outer side of the axial direction of the tire and to be convexly shaped, as a result, the bead portions connected with the sidewall are obliged to undergo a large flexure, thus, as previously stated, the "blow out" phenomenon of the carcass ply cords is accelerated.
Therefore, to control the expansion of the cord path is the most important role in a matter of controlling the flexture in the bead portions, thus the second reinforcing layer composed of at least two organic fiber cord reinforcing plies is arranged in the inner side of the carcass ply main portion in order to control the phenomenon of "expansion of cord paths", and an apex triangle composed of a single rubber stock having low modulus and low heat generating characteristics is disposed between the carcass ply main portion and the turnup portion of the carcass ply, in order to improve the resistance to the dynamic and thermal fatigue, by means of controlling the heat generation in the bead regions. As a result, there is provided a highly durable reinforced construction of the bead regions of large size radial tires for truck and other heavy vehicles which can be recapped two or three times without damaging the carcass and with a minimum of the "blow out" phenomenon of the carcass ply.